FIG. 1 is a diagram of a sensor 1 of this kind, with a closed magnetic circuit 2 on elongate branches 3 whereof are wound excitation coils 4 and a detection coil 5.
The sensor is fabricated using the standard microtechnology techniques. Thus the magnetic circuit comprises a soft magnetic material deposited in thin layers (in particular: Permalloy®, amorphous or other material, and the like). The excitation and detection coils are thin layers of a conductive material such as aluminum, copper, gold, and the like. These coils may be solenoidal or planar spirals in particular.
The detection circuit collects the magnetic flux produced by the soft magnetic material when excited by the current flowing in the excitation circuit(s).
The coils may be interleaved.
There may be a supplementary coil to apply a field for compensating the DC or low-frequency magnetic field to be measured. The detection coil may also be used for this purpose, however.
In practice, the signal necessitates an area of saturation of the magnetic material and, in the situation where saturation is achieved, at least in part, the magnetic flux collected by the detection coil no longer has the same alternations in the presence of the DC field to be measured. The detection signal is the derivative of this flux.
This dissymmetry is reflected in the appearance of a second order harmonic in the detection signal linked to the DC field to be measured.
To enhance the measured signal, two detection circuits may be connected in differential mode, in which case the detection signal has twice the frequency of the excitation signal.
The miniature sensors known as “microflux gates” are typically intended for measuring DC or low-frequency magnetic fields (or magnetic field variations) of the order of a few nanoteslas, at the present time, and in a range of approximately +/−100 microteslas. They are used in particular to detect very small variations in the terrestrial magnetic field.
The fact that microflux gates are small has the advantages of great lightness, small bulk (which is beneficial in aerospace and medical applications, in various industrial applications, in clamp ammeters, and the like) and a low fabrication cost, given the use of collective fabrication techniques using the magnetic microelectronic technology.
Examples of microflux gate integrated components are described in the following documents in particular:
“High directional sensitivity of micromachined magnetic fluxgate sensors” by RAHMAN A. RUB, SUKIRTI GUPTA, and CHONG H. AHN—University of Cincinnati, Ohio, 45221-0030-USA, Transducers '01 EUROSENSORS XV-2001,
“Performance and applications of a two axes fluxgate magnetic field sensor fabricated by a CMOS process” by H. GRÜGER, R. GOTTFRIED-GOTTFRIED—Fraunhofer Institute for microelectronic circuits and systems IMS Dresden, Germany—Sensor and Actuators A 91 (2001) 61–64,
“Micro fluxgate magnetic sensor interface circuits using deltaS Modulation” Shuji KOGA, Akira YAMASAWA, Shoji KAWAHITO—Toyohashi Univ of Technology—T IEE Japan, vol 117-E, N° 2, (1997),
“A miniaturized magnetic-field sensor system consisting of a planar fluxgate sensor and a CMOS readout circuitry” by R. GOTTFRIED-GOTTFRIED, W. BUDDE, R. JÄHNE, H. KÜCK—Fraunhofer Institute for microelectronic circuits and systems IMS Dresden, Germany—Sensor and Actuators A54 (1996) 443–447.
The circuits described therein are either closed (looped) or open with parallelepiped-shaped bars.
However, the detection of very weak magnetic fields is in practice prevented by problems of instability or offsets (the expression “offset jumps” is sometimes used). The measurement time signal could theoretically have a noise level of the order of 1 nanotesla, but is in practice greatly degraded by the presence of instabilities or jumps, which can be of the order of 100 to 1000 nanoteslas. These jumps occur at a frequency of a few Hz, but also at a frequency of one per second, one per minute, one per hour or even one per day.